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Tips on removing noise from digital image

From:    bucci@v...
Date:    Wed Aug 9, 2000 1:12pm
Subject: Repairing iso 400 Images
Origin:  Olympus_C2500L on eGroups

Does anyone know how to repair a digital image in Photoshop with excessive noise? I have a few images which I shot in the ISO 400 mode which are extremely noisy when printed out on my epson. Any information on repairing these images would be greatly appreciated.

Thanks for your help.

From:    Chieh Cheng
Date:    Wed Aug 9, 2000 5:51pm
Subject: Re: Repairing iso 400 Images
Origin:  Olympus_C2500L on eGroups

Try using the Dust & Scratch filter in Photoshop. Set the Radius setting to 1 pixel would probably smooth out all the noise. Keep in mind that this filter will soften your image slightly.


From:    Christian Kuiphoff
Date:    Wed Aug 9, 2000 11:38pm
Subject: Re: Re: Repairing iso 400 Images
Origin:  Olympus_C2500L on eGroups

If you do this with an adjustment layer, you could use the history brush to paint back in the non-noisy areas and then flatten the image when satisfied.


From:    Elaine Pawelko
Date:    Wed Aug 9, 2000 1:35pm
Subject: Re: Repairing iso 400 Images
Origin:  Olympus_C2500L on eGroups


Unfortunately, eGroups has not updated their site so I can't address you properly by name. I would still like to take this opportunity to welcome you to our group.

A very quick solution would be to use the noise filters in Qimage. BUT, I would really like to discuss the techniques of reducing noise (all ISO settings and long exposure) in Photoshop as I feel it's a far better way to deal with this situation. Possibly, maybe we can list a general outline of photo repair for a soft mode photo: what order should we repair photos--levels &/or curves, unsharp mask, noise filters, etc. Just something to get us all started for those that feel intimidate using this program and others like it. As always, this guideline can be used with other image editing software programs; maybe just the terminology is different.


From:    gordoncheung@h...
Date:    Wed Aug 9, 2000 10:15pm
Subject: Re: Repairing iso 400 Images (noise buster, LONG)
Origin:  Olympus_C2500L on eGroups


Ever since the last time noise was discussed heavily in this group, I've been accumulating a sizable collection of noise removal actions. Each action is designed to fight a specific kind of noise. There are too many to list here but I think I can summerize my approach and maybe others can comment on it.

First let's talk about noise. The best way to remove noise is not to create them in the first place. To me, it means ISO100 as much as possible and don't underexpose by too much (I do want to remind everybody about erring on the underexposure side still holds, just don't overdo it). When contrast is high, use fill flash or reflector if possible.

I group noise into 2 catagories, shadow noise and long exposure noise. Shadow noise is a general low level noise that spreads out in dark areas. Long exposure noise happens in shutter speed slower than .5s and looks like bright sparkles of individual pixels. Note that shadow noise could happen in brightly lit photos, it could lurk in dark shadows, or one of the other color channels (for example, blue channel on a red subject). Once you identify the type of noise, then you can pick the right tool for the job.

My general approach to noise fixing is as follow: 1. global fixes - do level/curve, color balance fix first. These operations exaggerate filtering artifacts if done after filtering, so it's best to be done first.

2. isolation - try to isolate the noisy area so the rest of the picture retains its original quality. All noise filters soften the picture to some degree, it's best to apply it to only where it's needed. How to isolate? for shadow noise, you can use color select to mask out the brighter area and leave only the shadows. For long exposure noise, use the threshold adjustment in Dust and Scratches filter to isolate the salt and pepper. Once you identify the noise, then you can find the right tool for the job.

3. filter - use filter to remove noise. Dust and scratches filter is a median filter with a threshold setting. It's a non-linear filter that does a little better in terms of removing noise without too much softening. Gaussian filter is another popular choice. It's best for occations where bluring is not a problem.

4. sharpen - since all filters soften pictures, sharpening is a must after filtering. Unsharp Mask does the best for sharpening with minimal amount of typical filter artifacts. Keep the setting low and run multiple passes to boost edges and maintain smooth textures.

5. crop and print - not required, but a well cropped picture can often save the composition of a photo. Afterall, you are only looking at 95% of the picture when you frame it on the view finder. Pictures that I care enough to print, I would like to try to crop and improve the composition.

This is my 5 steps program, what's yours?


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